RAMs such as SRAMs require backup batteries to which they are constantly connected in order to maintain the data stored therein, although the current drawn from such backup batteries is very small, e.g. 50 ua. NiCd batteries are typically used as the backup batteries. As is well known, such rechargeable batteries lose charge even when current is not drawn from them. Consequently, in a computer environment they must be continuously charged, typically at a rate of about 5 to 10 ma. This continuous high charge rate has been found to limit the lifetimes of such batteries.
Under some conditions, substantially more current is required to be drawn from the battery, and sometimes the charging voltage rail drops in voltage. This limits the charging current, and at the same time can reduce the amount of current that can be drawn by the RAM, endangering the data. Prior art RAM backup battery charging circuits have been unable to compensate for these deficiencies, at reasonable cost.